This invention relates to methods and apparatus for calibrating temperature measurements, e.g., during rapid thermal processing of substrates, and for measuring currents.
In rapid thermal processing (RTP), a substrate is heated quickly to a high temperature, such as 1200.degree. C., to perform one or more fabrication steps such as annealing, cleaning, chemical vapor deposition, oxidation, or nitridation. To obtain high yields and process reliability in the manufacture of submicron devices, the temperature of the substrate during such fabrication steps must be precisely controlled. For example, to fabricate a dielectric layer with a thickness of 60-80 .ANG. and a uniformity of .+-.2 .ANG., the temperature in successive thermal processing runs should not vary by more than a few degrees centigrade (.degree. C.) from the target temperature. To achieve this level of temperature control, the temperature of the substrate should be measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. An optical pyrometer using an optical probe samples the emitted radiation intensity from the substrate, and computes the temperature of the substrate based on the spectral emissivity of the substrate and the ideal black body radiation-temperature relationship given by Planck's law: ##EQU1##
where C.sub.1 and C.sub.2 are known constants, .lambda. is the radiation wavelength of interest, and T is the substrate temperature measured in degrees Kelvin (.degree. K). The spectral emissivity .epsilon.(.lambda.,T) of an object is the ratio of its emitted spectral intensity .PSI.(.lambda.,T) to the spectral intensity .PSI..sub.B (.lambda.,T) of a black body at the same temperature. That is, ##EQU2##
Since C.sub.1 and C.sub.2 are known constants, under ideal conditions, the temperature of the substrate can be accurately determined if .epsilon.(.lambda.,T) is known.
When the temperature measurement system is first installed into the RTP system, the optical probe must be calibrated so that it produces an accurate temperature reading when exposed to the radiation emitted by the heated substrate. The temperature measurement system also must be periodically recalibrated because the temperature sensed by the probe may shift over time. Such temperature measurement shifts may be caused by, e.g., contamination of or damage to the light pipe that is used to sample the emitted radiation being emitted from the substrate, or by drifts in the electronic components of the pyrometer.